Rotation in xy plane to suppress spurious modes in xbar devices

ABSTRACT

Acoustic resonator devices, filter devices, and methods of fabrication are disclosed. An acoustic resonator includes a substrate having a surface and a single-crystal piezoelectric plate having front and back surfaces. The back surface is attached to the surface of the substrate except for a portion of the piezoelectric plate forming a diaphragm that spans a cavity in the substrate. An interdigital transducer (IDT) is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm. The IDT is configured to excite a primary acoustic mode in the diaphragm in response to a radio frequency signal applied to the IDT. The interleaved fingers extend at an oblique angle to an Z crystalline axis of the piezoelectric plate.

RELATED APPLICATION INFORMATION

This patent claims priority from provisional patent application62/896,821, filed Sep. 6, 2019, entitled ROTATION IN XY PLANE TOSUPPRESS SPURIOUS MODES IN XBAR DEVICES, the entire content of which isincorporated herein by reference.

This patent is also a continuation in part of application Ser. No.16/689,707, entitled BANDPASS FILTER WITH FREQUENCY SEPARATION BETWEENSHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYER THICKNESS, filedNov. 20, 2019, which claims priority from application Ser. No.16/230,443, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR,filed Dec. 21, 2018, now U.S. Pat. No. 10,491,192, issued Nov. 26, 2019,which claims priority from the following provisional patentapplications: application 62/685,825, filed Jun. 15, 2018, entitledSHEAR-MODE FBAR (XBAR); application 62/701,363, filed Jul. 20, 2018,entitled SHEAR-MODE FBAR (XBAR); application 62/741,702, filed Oct. 5,2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE FILMBULK ACOUSTIC RESONATOR, and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. The currentLTE™ Long Term Evolution) specification defines frequency bands from 3.3GHz to 5.9 GHz. Some of these bands are not presently used. Futureproposals for wireless communications include millimeter wavecommunication bands with frequencies up to 28 GHz.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave BAW) resonators, film bulk acousticwave resonators (FBAR), and other types of acoustic resonators. However,these existing technologies are not well-suited for use at the higherfrequencies proposed for future communications networks.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1.

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1.

FIG. 3B is another alternative schematic cross-sectional view of theXBAR of FIG. 1.

FIG. 3C is an alternative schematic plan view of an XBAR.

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 5 is a graphical representation of Euler angles.

FIG. 6 is a schematic plan view of the XBAR of FIG. 1 showing an X-axisand a Y-axis of a piezoelectric plate.

FIG. 7 is a chart of piezoelectric constants as a function ofinterdigital transducer angle for Z-cut LiNbO3.

FIG. 8A is a graph of the magnitude of admittance versus frequency foran XBAR where fingers of an interdigital transducer extend in adirection parallel to an X-axis of a piezoelectric plate.

FIG. 8B is a graph of the magnitude of admittance versus frequency foran XBAR where fingers of an interdigital transducer are at an angletheta of 25 degrees with respect to an X-axis of a piezoelectric plate.

FIG. 9A is an enlarged view the resonance frequencies of FIG. 8A andFIG. 8B overlaid.

FIG. 9B is an enlarged view the anti-resonance frequencies of FIG. 8Aand FIG. 8B overlaid.

FIG. 10A is a graph of the magnitude of admittance versus frequency foranother XBAR where fingers of an interdigital transducer extend at anangle theta of 90 degrees to an X-axis of a piezoelectric plate.

FIG. 10B is a chart of spur magnitude as a function of angle theta forthe XBAR of FIG. 10A.

FIG. 11A is a graph of the magnitude of admittance versus frequency forXBARs with various angles theta of rotation to the X-axis of thepiezoelectric plate.

FIG. 11B is an enlarged view of a portion of FIG. 11A around a frequencyof 2 GHz.

FIG. 11C is an enlarged view of a portion of FIG. 11A around a frequencyof 3 GHz.

FIG. 11D is an enlarged view of a portion of FIG. 11A around a frequencyof 4.4 GHz.

FIG. 11E is an enlarged view of a portion of FIG. 11A around a frequencyof 4.7 GHz.

FIG. 11F is an enlarged view of a portion of FIG. 11A around a resonancefrequency.

FIG. 11G is an enlarged view of a portion of FIG. 11A around ananti-resonance frequency.

FIG. 12A is a graph of the magnitude of admittance versus frequency forXBARs with different angles theta of rotation to the X-axis of thepiezoelectric plate.

FIG. 12B is an enlarged view of a portion of FIG. 12A around a resonancefrequency.

FIG. 13 is a schematic circuit diagram and layout of a filter usingXBARs.

FIG. 14 is a flow chart of a process for fabricating an XBAR.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. As will be discussed later in further detail, thepiezoelectric plate is cut such that the orientation of the X, Y, and Zcrystalline axes with respect to the front and back surfaces is knownand consistent. In the examples presented in this patent, thepiezoelectric plates are Z-cut, which is to say the Z axis is normal tothe surfaces. However, XBARs may be fabricated on piezoelectric plateswith other crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asubstrate 120 that provides mechanical support to the piezoelectricplate 110. The substrate 120 may be, for example, silicon, sapphire,quartz, or some other material. The piezoelectric plate 110 may bebonded to the substrate 120 using a wafer bonding process, or grown onthe substrate 120, or attached to the substrate in some other manner.The piezoelectric plate may be attached directly to the substrate, ormay be attached to the substrate via one or more intermediate materiallayers.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites an acoustic wave within thepiezoelectric plate 110. As will be discussed in further detail, theexcited acoustic wave is a bulk shear wave that propagates in thedirection normal to the surface of the piezoelectric plate 110, which isalso normal, or transverse, to the direction of the electric fieldcreated by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

A cavity 125 is formed in the substrate 120 such that the portion of thepiezoelectric plate 110 containing the IDT 130 is suspended over thecavity 125 without contacting the substrate 120. “Cavity” has itsconventional meaning of “an empty space within a solid body.” The cavity125 may be a hole completely through the substrate 120 (as shown inSection A-A and Section B-B) or a recess in the substrate 120. Thecavity 125 may be formed, for example, by selective etching of thesubstrate 120 before or after the piezoelectric plate 110 and thesubstrate 120 are attached. As shown in FIG. 1, the cavity 125 has arectangular shape with an extent greater than the aperture AP and lengthL of the IDT 130. A cavity of an XBAR may have a different shape, suchas a regular or irregular polygon. The cavity of an XBAR may more orfewer than four sides, which may be straight or curved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers is greatly exaggerated with respect to the length (dimensionL) and aperture (dimension AP) of the XBAR. A typical XBAR has more thanten parallel fingers in the IDT 110. An XBAR may have hundreds, possiblythousands, of parallel fingers in the IDT 110. Similarly, the thicknessof the fingers in the cross-sectional views is greatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2, the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. tfd and tbd maybe, for example, 0 to 500 nm. tfd and tbd are typically less than thethickness ts of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The IDT fingers 238 may be aluminum or a substantially aluminum alloy,copper or a substantially copper alloy, molybdenum, beryllium, gold, orsome other conductive material. Thin (relative to the total thickness ofthe conductors) layers of other metals, such as chromium or titanium,may be formed under and/or over the fingers to improve adhesion betweenthe fingers and the piezoelectric plate 110 and/or to passivate orencapsulate the fingers. The busbars (132, 134 in FIG. 1) of the IDT maybe made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2 to 20 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2 to 20 timesthe thickness ts of the piezoelectric slab 212. The width of the IDTfingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1) of the IDT may be the same as, or greater than, the thickness tmof the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1. In FIG. 3A, a piezoelectric plate310 is attached to a substrate 320. A cavity 325, which does not fullypenetrate the substrate 320, is formed in the substrate under theportion of the piezoelectric plate 310 containing the IDT of an XBAR.The cavity 325 may be formed, for example, by etching the substrate 320before attaching the piezoelectric plate 310. Alternatively, the cavity325 may be formed by etching the substrate 320 with a selective etchantthat reaches the substrate through one or more openings provided in thepiezoelectric plate 310.

In FIG. 3B, the substrate 320 includes a base 322 and an intermediatelayer 324 disposed between the piezoelectric plate 310 and the base 322.For example, the base 322 may be silicon and the intermediate layer 324may be silicon dioxide or silicon nitride or some other material. Acavity 325 is formed in the intermediate layer 324 under the portion ofthe piezoelectric plate 310 containing the IDT of an XBAR. The cavity325 may be formed, for example, by etching the intermediate layer 324before attaching the piezoelectric plate 310. Alternatively, the cavity325 may be formed by etching the intermediate layer 324 with a selectiveetchant that reaches the substrate through one or more openings providedin the piezoelectric plate 310.

FIG. 3C is a schematic plan view of another XBAR 350. The XBAR 350includes an IDT formed on a piezoelectric plate 310. The piezoelectricplate 310 is disposed over a cavity 380 in a substrate. In this example,the cavity 380 has an irregular polygon shape such that none of theedges of the cavity are parallel, nor are they parallel to theconductors of the IDT. A cavity may have a different shape with straightor curved edges.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. An RF voltage is applied to the interleaved fingers 430. Thisvoltage creates a time-varying electric field between the fingers. Thedirection of the electric field is lateral, or parallel to the surfaceof the piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric field is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites shear-mode acoustic waves, in thepiezoelectric plate 410. In this context, “shear deformation” is definedas deformation in which parallel planes in a material remain paralleland maintain a constant distance while translating relative to eachother. “Shear acoustic waves” are defined as acoustic waves in a mediumthat result in shear deformation of the medium. The shear deformationsin the XBAR 400 are represented by the curves 460, with the adjacentsmall arrows providing a schematic indication of the direction andmagnitude of atomic motion. The degree of atomic motion, as well as thethickness of the piezoelectric plate 410, have been greatly exaggeratedfor ease of visualization. While the atomic motions are predominantlylateral (i.e. horizontal as shown in FIG. 4), the direction of acousticenergy flow of the excited shear acoustic waves is substantiallyvertical, normal to the surface of the piezoelectric plate, as indicatedby the arrow 465.

Considering FIG. 4, there is little electric field strength immediatelyunder the IDT fingers 430, and thus acoustic modes are only minimallyexcited in the regions 470 under the fingers. There may be evanescentacoustic motions in these regions. An acoustic resonator based on shearacoustic wave resonances can achieve better performance than currentstate-of-the art film-bulk-acoustic-resonators (FBAR) andsolidly-mounted-resonator bulk-acoustic-wave (SMR BAW) devices where theelectric field is applied in the thickness direction. In such devices,the acoustic mode is compressive with atomic motions and the directionof acoustic energy flow in the thickness direction. In addition, thepiezoelectric coupling for shear wave XBAR resonances can be high (>20%)compared to other acoustic resonators. Thus, high piezoelectric couplingenables the design and implementation of microwave and millimeter-wavefilters with appreciable bandwidth.

FIG. 5 is a graphical illustration of Euler angles. Euler angles are asystem, introduced by Swiss mathematician Leonhard Euler, to define theorientation of a body with respect to a fixed coordinate system. Theorientation is defined by three successive rotations about angles α, β,and γ.

As applied to acoustic wave devices, XYZ is a three-dimensionalcoordinate system aligned with the crystalline axes of the piezoelectricmaterial. xyz is a three-dimensional coordinate system aligned with theacoustic wave device, where the z axis is normal to the surface of thepiezoelectric material and xy is the plane of the surface of thepiezoelectric material. The vector N is the intersection of the xy andXY planes. The vector N is also the common perpendicular to the z and Zaxis.

Although application Ser. No. 16/230,443 is not limited to a specifictype or orientation of a piezoelectric material, all of the examples inthose applications use Lithium Tantalate or Lithium Niobatepiezoelectric plates with the Z crystalline axis normal to the platesurface and the Y crystalline axis orthogonal to the IDT fingers. Suchpiezoelectric plates have Euler angles of 0°, 0°, 90°. Relatedapplication Ser. No. 16/518,594 describes XBAR devices on piezoelectricplates with Euler angles 0°, β, 90°, where −15°≤β<0°. Such XBAR devicesalso have higher piezoelectric coupling than devices on piezoelectricplates with Euler angles of 0°, 0°, 90°. Related application Ser. No.16/782,971 describes XBAR devices on piezoelectric plates with Eulerangles 0°, β, 0°, where 0°≤β<60°. Such XBAR devices have higherpiezoelectric coupling than devices on piezoelectric plates with Eulerangles of 0°, 0°, 90°. In all of these XBAR configurations, one or bothof crystalline X and Y axes lie in the plane of the surface, and thefinger of the IDT are perpendicular to either the X or Y crystallineaxis.

The IDT of an XBAR may excite other undesired, or spurious, acousticmodes that may include, but are not limited to, harmonics and higherorders of the primary acoustic mode, plate modes that propagateorthogonal to the IDT fingers, and transverse modes that propagateparallel to the IDT fingers. These spurious acoustic modes may causeundesired perturbations or spurs in the admittance characteristics of anXBAR.

The dominant parameter that determines the resonance and anti-resonancefrequencies of the primary mode of an XBAR is the thickness of thepiezoelectric membrane. The resonance and anti-resonance frequenciesalso depend, to a lesser extent, on the pitch and width, or mark, of theIDT fingers. The resonance and anti-resonance frequencies are almosttotally independent of the rotation of the XBAR device in the X-Y plane.However, many of the propagating spurious modes are sensitive to thisangle of rotation. Thus, rotation in the X-Y plane provides a method tosuppress at least some spurious modes without materially effecting theprimary acoustic mode of the XBAR.

FIG. 6 shows a schematic plan view of an XBAR using a Z-cutpiezoelectric plate where the X-axis 680 and the Y-axis 690 of thepiezoelectric plate 110 are indicated. As can be seen in FIG. 6, the IDT130 is rotated with respect to the X-axis 680 and the Y-axis 690 suchthat the fingers 136 of the IDT 130 extend in an oblique direction thatis neither parallel nor orthogonal to the X-axis 680 or the Y-axis 690.In an example, the fingers 136 of the IDT 130 could be at an angle gammaγ in a range from 60 degrees to 120 degrees to the Y-axis. The angle canbe selected so as to maximally suppress problematic spurs. In anotherexample, the fingers 136 could be at other angles to the X-axis, Y-axis,or the Z-axis.

FIG. 7 is a chart 700 of piezoelectric constants of Z-cut LiNbO3 as afunction of degrees of the angle gamma. The piezo electric constants(represented by the matrix e_(ij)) determine how strongly the electricfield couples to the displacements of various acoustic modes. Thedot-dash curve 710 is piezoelectric coefficient e15, which isindependent of the angle gamma. e15 and determines the coupling to theprimary XBAR mode. The solid curve 720 is e11, which determines couplingto propagating S0 modes. The dashed curve 730 is e12, which determinescoupling to propagating SH0 modes. Vertical line 740 at 90 degreesindicates the nominal XBAR, as described in U.S. Pat. No. 10,491,192.Because of the crystal symmetry (120 degrees rotation) and the fact thatthe sign of the piezoelectric coefficient is not relevant here todetermining mode coupling, only angles from 60 to 120 degrees need to beconsidered.

FIG. 8A is a chart 800 with a curve 810 plotting the magnitude of theadmittance of an exemplary XBAR as a function of frequency where fingersof the IDT extend in a direction parallel to the X-axis of apiezoelectric plate (i.e., gamma=90). The curve 810 exhibits a resonance(where the admittance is highest) at a frequency about 4700 MHz and ananti-resonance (where the admittance is lowest) at a frequency about5300 MHz. Spurious modes 820, 822, 824, and 826 are evident in theadmittance curve. Such spurious modes can result in undesired effectssuch as increased insertion loss and transmission ripple in the passbandof a filter.

FIG. 8B is a chart 850 with a curve 860 plotting the magnitude of theadmittance of an exemplary XBAR as a function of frequency where thefingers of the IDT are rotated to an angle gamma of 65 degrees withrespect to an X-axis of a piezoelectric plate.

FIG. 9A is a chart 900 with an enlarged view the resonance frequenciesof FIG. 8A and FIG. 8B overlaid. FIG. 9B is a chart 950 with an enlargedview the anti-resonance frequencies of FIG. 9A and FIG. 9B overlaid.

Both the XBAR of FIG. 8A and the XBAR of FIG. 8B have piezoelectricplates formed of ZX-cut lithium niobate and IDTs formed of aluminum, andp=5 um, mark=650 nm, tm=100 nm, is =400 nm, tfd=0. Comparison of FIGS.8A and 8B shows that a rotation of 25 degrees in the X-Y planesubstantially reduces the spurious modes. The spurious modes 820, 822,824 seen in FIG. 8A are effectively suppressed and the amplitude of thespurious mode 862 is reduced relative to spurious mode 826. Theresonance frequency is only increased by 1 MHz (0.02%) and theanti-resonance frequency is only reduced by 1 MHz (0.02%), as seen inmore detail in FIGS. 9A and 9B. Rotation in the XY plane of the lithiumniobate piezoelectric with respect to the IDT has only a minimal effecton the primary XBAR mode.

The invention is not limited to a particular rotation. Rather, X-Yrotation provides a degree-of-freedom that a designer can use tosuppress spurious modes during the design of an XBAR filter withoutaltering the characteristics of the primary mode. By changing thein-plane rotation angle gamma, S0 modes can be suppressed at the expenseof exciting SH0 modes. Since S0 and SH0 modes have different velocities,they will in general lay at distinct frequencies such that one mayimpact important filter requirements more than another.

FIG. 10A is a chart 1000A of resonance and anti-resonance frequenciesshown in solid line curve 1005 of another exemplary XBAR where fingersof an interdigital transducer extend at an angle gamma of 90 degrees toa Y-axis of a piezoelectric plate. For this XBAR on ZX-cut LN with agamma=90 degrees, p=5 um, mark=650 nm, tm=100 nm, is =400 nm, and tfd=0.Areas of interest are identified at 1010A, 1020A, 1030A, 1040A, 1050A,and 1060A, including problematic spurs.

FIG. 10B is a chart of spur magnitude as a function of angle gamma forthe XBAR of FIG. 10A. Spur 1010A corresponds to curve 1010B, spur 1020Acorresponds to curve 1020B, spur 1030A corresponds to curve 1030B, spur1040A corresponds to curve 1040B, spur 1050A corresponds to curve 1050B,and spur 1060A corresponds to curve 1060B. In this example, gamma isvaried from the original 90 degrees shown in FIG. 10A. When gamma isvaried, spurs such as 1010B and 1050B due to S0 plate modes aresuppressed at gamma=60 degrees. Other spurs, such as 1020B and 1040B dueto SH0 plate modes are excited. Still other spurs, such as 1030B and1060B, due to A0 plate modes are substantially unaffected by therotation.

FIG. 11A is a chart 1100A of resonance and anti-resonance frequencies ofXBARs showing the magnitude of the admittance as a function of frequencyfor different angles gamma of rotation to the Y-axis of thepiezoelectric plate. The piezoelectric plate of FIG. 11A is formed ofZX-cut lithium niobite, has IDTs formed of aluminum, and has p=5 um,mark=650 nm, tm=100 nm, is =400 nm, and tfd=0. The XBAR represented bysolid curve 1110 has an angle gamma of 90 degrees. The XBAR representedby dash curve 1120 has an angle gamma of 75 degrees. The XBARrepresented by dot curve 1130 has an angle gamma of 60 degrees.

FIG. 11B is a chart 1100B showing an enlarged view of a portion of FIG.11A around a frequency of 2 GHz. FIG. 11C is a chart 1100C showing anenlarged view of a portion of FIG. 11A around a frequency of 3 GHz. FIG.11D is a chart 1100D showing an enlarged view of a portion of FIG. 11Aaround a frequency of 4.4 GHz. FIG. 11E is a chart 1100E showing anenlarged view of a portion of FIG. 11A around a frequency of 4.7 GHz.These charts show that varying the angle gamma can allow a designer tosuppress spurs in certain frequency ranges or move spurs to moredesirable frequencies where they cause fewer problems.

FIG. 11F is a chart 1100F showing an enlarged view of a portion of FIG.11A around a resonance frequency. FIG. 11G is a chart 1100G showing anenlarged view of a portion of FIG. 11A around an anti-resonancefrequency. These charts show that resonance and antiresonancefrequencies can be substantially unaffected by variation in angle gamma.

FIG. 12A is a chart 1200A of resonance and anti-resonance frequencies ofother exemplary XBARs showing the magnitude of the admittance as afunction of frequency for different angles gamma of rotation to theX-axis of the piezoelectric plate. For this XBAR on ZX-cut LN, p=3.5 um,mark=500 nm, tm=100 nm, is =400 nm, and tfd=0. The XBAR represented bysolid curve 1210 has an angle gamma of 90 degrees. The XBAR representedby dash curve 1220 has an angle gamma of 75 degrees. The XBARrepresented by dot curve 1230 has an angle gamma of 60 degrees. FIG. 12Bis a chart 1200B showing an enlarged view of a portion of FIG. 12Aaround a resonance frequency. These charts show that varying the anglegamma can allow a designer to suppress spurs in certain frequency rangeswithout substantially varying a resonance frequency.

Acoustic RF filters usually incorporate multiple acoustic resonators.Typically, these resonators have at least two different resonancefrequencies. For example, an RF filter using the well-known “ladder”filter architecture includes shunt resonators and series resonators. Ashunt resonator typically has a resonance frequency below the passbandof the filter and an anti-resonance frequency within the passband. Aseries resonator typically has a resonance frequency within the passband and an anti-resonance frequency above the passband. In manyfilters, each resonator has a unique resonance frequency. An ability toobtain different resonance frequencies for XBARs made on the samepiezoelectric plate greatly simplifies the design and fabrication of RFfilters using XBARs.

FIG. 13 is a schematic circuit diagram and layout for a high frequencyband-pass filter 1300 using XBARs. The filter 1300 has a conventionalladder filter architecture including three series resonators 1310A,1310B, 1310C and two shunt resonators 1320A, 1320B. The three seriesresonators 1310A, 1310B, and 1310C are connected in series between afirst port and a second port. In FIG. 13, the first and second ports arelabeled “In” and “Out”, respectively. However, the filter 1300 issymmetrical and either port and serve as the input or output of thefilter. The two shunt resonators 1320A, 1320B are connected from nodesbetween the series resonators to ground. All the shunt resonators andseries resonators are XBARs. In the example of FIG. 13, the two shuntresonators 1320A, 1320B are rotated with respect to the seriesresonators 1310A, 1310B, 1310C. One, some, or all of the resonators in afilter may rotated at the same or different angles. The ability torotate the resonators independently gives the filter designer anadditional degree of design freedom to minimize spurious effects.

The three series resonators 1310A, B, C and the two shunt resonators1320A, B of the filter 1300 are formed on a single plate 930 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 13, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 1335). In this example, each IDT is disposed over arespective cavity. In other filters, the IDTs of two or more resonatorsmay be disposed over a single cavity.

Description of Methods

FIG. 14 is a simplified flow chart showing a process 1400 for making anXBAR or a filter incorporating XBARs. The process 1400 starts at 1405with a substrate and a plate of piezoelectric material and ends at 1495with a completed XBAR or filter. The flow chart of FIG. 14 includes onlymajor process steps. Various conventional process steps (e.g. surfacepreparation, cleaning, inspection, baking, annealing, monitoring,testing, etc.) may be performed before, between, after, and during thesteps shown in FIG. 14.

The flow chart of FIG. 14 captures three variations of the process 1400for making an XBAR which differ in when and how cavities are formed inthe substrate. The cavities may be formed at steps 1410A, 1410B, or1410C. Only one of these steps is performed in each of the threevariations of the process 1400.

The piezoelectric plate may be, for example, Z-cut lithium niobate orlithium tantalate as used in the previously presented examples. Thepiezoelectric plate may be some other material and/or some other cut.The substrate may preferably be silicon. The substrate may be some othermaterial that allows formation of deep cavities by etching or otherprocessing.

In one variation of the process 1400, one or more cavities are formed inthe substrate at 1410A, before the piezoelectric plate is bonded to thesubstrate at 1420. A separate cavity may be formed for each resonator ina filter device. The one or more cavities may be formed usingconventional photolithographic and etching techniques. Typically, thecavities formed at 1410A will not penetrate through the substrate, andthe resulting resonator devices will have a cross-section as shown inFIG. 3A or FIG. 3B.

At 1420, the piezoelectric plate is bonded to the substrate. Thepiezoelectric plate and the substrate may be bonded by a wafer bondingprocess. Typically, the mating surfaces of the substrate and thepiezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the substrate. One or both mating surfaces may be activatedusing, for example, a plasma process. The mating surfaces may then bepressed together with considerable force to establish molecular bondsbetween the piezoelectric plate and the substrate or intermediatematerial layers.

A conductor pattern, including IDTs of each XBAR, is formed at 1430 bydepositing and patterning one or more conductor layer on the front sideof the piezoelectric plate. The conductor layer may be, for example,aluminum, an aluminum alloy, copper, a copper alloy, or some otherconductive metal. Optionally, one or more layers of other materials maybe disposed below (i.e. between the conductor layer and thepiezoelectric plate) and/or on top of the conductor layer. For example,a thin film of titanium, chrome, or other metal may be used to improvethe adhesion between the conductor layer and the piezoelectric plate. Aconduction enhancement layer of gold, aluminum, copper or other higherconductivity metal may be formed over portions of the conductor pattern(for example the IDT bus bars and interconnections between the IDTs).

The conductor pattern may be formed at 1430 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, and other etching techniques.

Alternatively, the conductor pattern may be formed at 1430 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 1440, a front-side dielectric layer may be formed by depositing oneor more layers of dielectric material on the front side of thepiezoelectric plate. The one or more dielectric layers may be depositedusing a conventional deposition technique such as sputtering,evaporation, or chemical vapor deposition. The one or more dielectriclayers may be deposited over the entire surface of the piezoelectricplate, including on top of the conductor pattern. Alternatively, one ormore lithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate, such as only between the interleaved fingers of theIDTs. Masks may also be used to allow deposition of differentthicknesses of dielectric materials on different portions of thepiezoelectric plate.

In a second variation of the process 1400, one or more cavities areformed in the back side of the substrate at 1410B. A separate cavity maybe formed for each resonator in a filter device. The one or morecavities may be formed using an anisotropic or orientation-dependent dryor wet etch to open holes through the back-side of the substrate to thepiezoelectric plate. In this case, the resulting resonator devices willhave a cross-section as shown in FIG. 1.

In the second variation of the process 1400, a back-side dielectriclayer may be formed at 1050. In the case where the cavities are formedat 1410B as holes through the substrate, the back-side dielectric layermay be deposited through the cavities using a convention depositiontechnique such as sputtering, evaporation, or chemical vapor deposition.

In a third variation of the process 1400, one or more cavities in theform of recesses in the substrate may be formed at 1410C by etching thesubstrate using an etchant introduced through openings in thepiezoelectric plate. A separate cavity may be formed for each resonatorin a filter device. The one or more cavities formed at 1410C will notpenetrate through the substrate, and the resulting resonator deviceswill have a cross-section as shown in FIG. 3A or FIG. 3B.

In all variations of the process 1400, the filter device is completed at1460. Actions that may occur at 1460 include depositing anencapsulation/passivation layer such as SiO₂ or Si₃O₄ over all or aportion of the device; forming bonding pads or solder bumps or othermeans for making connection between the device and external circuitry;excising individual devices from a wafer containing multiple devices;other packaging steps; and testing. Another action that may occur at1460 is to tune the resonant frequencies of the resonators within thedevice by adding or removing metal or dielectric material from the frontside of the device. After the filter device is completed, the processends at 1495.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: a substratehaving a surface; a single-crystal piezoelectric plate having front andback surfaces, the back surface attached to the surface of the substrateexcept for a portion of the piezoelectric plate forming a diaphragm thatspans a cavity in the substrate; and an interdigital transducer (IDT)formed on the front surface of the single-crystal piezoelectric platesuch that interleaved fingers of the IDT are disposed on the diaphragm,the IDT configured to excite a primary acoustic mode in the diaphragm inresponse to a radio frequency signal applied to the IDT, wherein thefingers of the IDT extend at an oblique angle to an Y crystalline axisof the piezoelectric plate.
 2. The device of claim 1, wherein adirection of acoustic energy flow of the excited primary shear acousticmode is substantially normal to the surfaces of the piezoelectric plate.3. The device of claim 1, wherein a z-axis of the piezoelectric plate isnormal to the front and back surfaces.
 4. The device of claim 1, whereinthe oblique angle is in a range between 60 degrees and 120 degrees. 5.The device of claim 1, wherein the piezoelectric plate is formed oflithium niobite.
 6. The device of claim 7, further comprising afront-side dielectric layer formed on the front surface of the lithiumniobate plate between the fingers of the IDT.
 7. The device of claim 7,wherein a thickness between the front and back surfaces of the lithiumniobate plate is greater than or equal to 200 nm and less than or equalto 1000 nm.
 8. A method of fabricating a filter device on apiezoelectric plate having front and back surfaces, the back surfaceattached to a substrate, the method comprising: forming one or morecavities in the substrate such that respective portions of thepiezoelectric plate form one or more diaphragms, each suspended over arespective cavity of the one or more cavities; forming a conductorpattern on the front surface, the conductor pattern including one ormore interdigital transducers (IDTs) of a respective one or moreresonators, wherein interleaved fingers of the one or more IDTs aredisposed on a respective diaphragm of the one or more diaphragms,wherein the piezoelectric plate and the one or more IDTs are configuredsuch that radio frequency signals applied to the one or more IDTs exciterespective primary shear acoustic modes in the respective one or morediaphragms, and wherein the interleaved fingers of at least one of theone or more IDTs extend at an oblique angle to an Y crystalline axis ofthe piezoelectric plate.
 9. The method of claim 8, wherein the obliqueangle is in a range between 60 degrees and 120 degrees.
 10. The methodof claim 8, wherein forming one or more cavities comprises forming aplurality of cavities to form a plurality of respective diaphragms,wherein the interleaved fingers of at least one of the IDTs disposed ona respective one of the plurality of diaphragms extend at an angledifferent from the interleaved fingers of at least one other of the IDTsdisposed on a respective diaphragm of the plurality of diaphragms. 11.The method of claim 8, wherein the one or more resonators includes ashunt resonator and a series resonator.
 12. The method of claim 8,wherein respective directions of acoustic energy flow of each of theexcited primary shear acoustic modes are substantially normal to thesurfaces of the piezoelectric plate.
 13. The method of claim 8, whereina z-axis of the piezoelectric plate is normal to the front and backsurfaces.
 14. A filter device comprising: a substrate having a surface;a single-crystal piezoelectric plate having front and back surfaces, theback surface attached to the surface of the substrate, portions of thepiezoelectric plate forming one or more diaphragms, each spanning arespective cavity of the one or more cavities in the substrate; and aconductor pattern formed on the front surface, the conductor patternincluding one or more interdigital transducers (IDTs) of a respectiveone or more resonators, wherein interleaved fingers of the one or moreIDTs is disposed on a respective diaphragm of the one or morediaphragms, wherein the piezoelectric plate and the one or more IDTs areconfigured such that respective radio frequency signals applied to theone or more IDTs excite respective shear primary acoustic modes in therespective one or more diaphragms, and wherein the interleaved fingersof at least one of the one or more IDTs extend at an oblique angle to anY crystalline axis of the piezoelectric plate.
 15. The filter device ofclaim 14, wherein the oblique angle is in a range between 60 degrees and120 degrees.
 16. The filter device of claim 14, wherein the one or morecavities comprise a plurality of cavities forming a plurality ofrespective diaphragms, wherein the interleaved fingers of at least oneof the IDTs disposed on a respective one of the plurality of diaphragmsextend at an angle different from the interleaved fingers of at leastone other of the IDTs disposed on a respective diaphragm of theplurality of diaphragms.
 17. The filter device of claim 14, wherein theone or more resonators includes a shunt resonator and a seriesresonator.
 18. The filter device of claim 14, wherein respectivedirections of acoustic energy flow of each of the excited primary shearacoustic modes are substantially normal to the surfaces of thepiezoelectric plate.
 19. The filter device of claim 14, wherein thepiezoelectric plate is formed of lithium niobite.
 20. The filter deviceof claim 14, wherein a z-axis of the piezoelectric plate is normal tothe front and back surfaces.